Polishing pads and methods relating thereto

ABSTRACT

A polishing pad having a uniform, continuously interconnected porous surface. The pad is produced by pressure sintering powder compacts of thermoplastic polymer at a temperature above the glass transition temperature but not exceeding the melting point of the polymer. The sintering process is conducted at a pressure in excess of 100 psi and in a mold having the desired final pad dimensions. In a preferred version, a mixture of two polymer powders is used, where one polymer has a lower melting point than the other.

This application is a continuation of application Ser. No. 08/814,514filed Mar. 10, 1997, now U.S. Pat. No. 6,106,754, which is acontinuation-in-part of application Ser. No. 08/782,717 filed Jan. 13,1997, now U.S. Pat. No. 6,017,265, which is a continuation-in-part ofapplication Ser. No. 08/480,166 filed Jun. 7, 1995 now abandoned, whichis a divisional of application Ser. No. 08/344,165 filed on Nov. 23,1994 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to polishing pads which areuseful in the polishing of semiconductor wafers, integrated circuits andthe like. More specifically, the polishing pads of the present inventionare directed to the use of sintered thermoplastic particles to enhancepolishing performance.

2. Description of Related Art

U.S. Pat. No. 3,763,054 discloses microporous polyurethane sheeting bymelt sintering sheets of loosely bonded particles.

U.S. Pat. No. 3,917,761 discloses a process of preparing porous sinteredpolyimide articles useful as oil filled bearings.

U.S. Pat. No. 4,256,845 discloses a method for manufacturing a porousthermoplastic sheet by gelling an aqueous latex dispersion of particlesand forming the dispersion into a sheet. This sheet is thenfree-sintered at a temperature at or above the melting point of thethermoplastic to form the final product.

U.S. Pat. No. 4,880,843 discloses a process for preparing a porousmolded composite article containing ultra high molecular weightpolyethylene and a polyethylene wax. The powder is melt sintered at atemperature in excess of the melting point of the polymer.

SUMMARY OF THE INVENTION

The present invention is directed to polishing pads comprising sinteredthermoplastic particles. The particles comprise a thermoplastic materialhaving a critical surface tension greater than or equal to 34milliNewtons per meter, a modulus of 1 to 200 megaPascals and anelongation to break in the range of 25% to 1000%. The particles arecompacted into a mold having the desired final pad dimensions at apressure in excess of 100 psi, and the particles are then sintered at atemperature above the glass transition temperature, but preferably belowthe melting point of the thermoplastic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a two-dimensional schematic drawing of packed sphericalparticles.

FIG. 2 is a cross-section drawing of a mold in an open position of atype which might be used to form a pad by the process of the presentinvention.

FIG. 3 is a cross-section drawing of the mold of FIG. 2 in the closedposition.

FIG. 4 is a graph of workpiece surface removal rate versus time in usefor a polishing pad of the prior art.

FIG. 5 is a graph of workpiece surface removal rate versus time in usefor a polishing pad of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to polishing pads derived from highpressure sintering of polymer powders, preferably at a temperature belowthe polymer's melting point. The sintering is preferably conducted in aprecisely shaped mold to provide a non-densified, porous material.

Thermoplastic polymers are generally viscoelastic, and theirtemperature/viscosity behavior can be complex. Polymer behavior over awide temperature range can be classified into three basic regions. Atlow temperatures, polymers behave as glassy, brittle solids, exhibitingpredominantly elastic behavior. The upper temperature boundary for thisregion is often referred to as the glass transition temperature or “Tg.”Above the Tg, but below the melting point of the polymer, viscouscharacteristics become more significant and polymers exhibit bothviscous and elastic effects. In this region, the polymer is capable ofconsiderable deformation when stress is applied. However, when thestress is removed, complete recovery may not occur, due to permanentmovement and rearrangement of the molecular structure of the polymer.Above the melting point, the polymer tends to behave as a viscousliquid, generally exhibiting permanent deformation when stress isapplied.

The processes of the present invention are preferably conducted belowthe melting point of the thermoplastic particulate material employed.Above the melting point of the material, rapid liquid sintering makesthe process difficult to control, particularly since a preciselyregulated and uniform pore structure is preferred. Also above themelting point, thermal gradients tend to cause variations in sinteringrate and can cause a non-uniform pore structure in the final article.Also, sintering above the polymer's melt temperature tends to cause: 1.unwanted adhesion between the molten polymer and the mold; and 2.unwanted deformation of the sintered product due to viscous flow.

In a preferred embodiment, the particles are sintered between the glasstransition temperature and the melting point of the particle material.Because the spontaneous rate of sintering is relatively low in thistemperature range, high pressures are preferably employed to increasethe sintering rate in a controllable fashion. Pressure is preferablyuniformly transmitted throughout the article to be compacted, so thesintering rate is essentially uniform throughout the bulk of thematerial. This largely eliminates porosity gradients in the finalproducts, making production of relatively thick articles possible.

By employing a mold or form of precisely pre-determined dimensions asthe means of imposing pressure, individual constituent polymer particlesare sintered in a precisely defined shape which corresponds to the molddimensions. This eliminates final shaping steps during manufacture andinhibits deformation of articles during the sintering process. Inaddition the lower temperature range largely prevents productdeformation and mold adhesion during sintering. Finally, the lowertemperatures eliminate or minimize thermal decomposition effects.Oxidative decomposition can generally be prevented by introducing inertgases into the mold prior to and/or during sintering.

Generally speaking, thermoplastics can be readily converted into apowder using conventional techniques, such as, cryogenic milling, andthe powdered thermoplastics will generally exhibit well defined thermalcharacteristics, including thermal stability as temperatures approachthe thermoplastic's melting point. The thermoplastic material can beselected according to hardness, elastic moduli, chemical durability, andabrasion resistance.

A wide variety of products may be produced using the same manufacturingequipment, simply by changing the process temperatures and pressures.Examples of thermoplastic polymers which might be used in the processesof the present invention are polyurethanes, polyamides, polycarbonates,polyacrylates (including methacrylates and acrylates), polysulfones, andpolyesters.

Preferably, the thermoplastic polymers of the present invention aresufficiently hydrophilic to provide a critical surface tension greaterthan or equal to 34 milliNewtons per meter, more preferably greater thanor equal to 37 and most preferably greater than or equal to 40milliNewtons per meter. Critical surface tension defines the wettabilityof a solid surface by noting the lowest surface tension a liquid canhave and still exhibit a contact angle greater than zero degrees on thatsolid. Thus, polymers with higher critical surface tensions are morereadily wet and are therefore more hydrophilic. Critical Surface Tensionof common polymers are provided below:

Polymer Critical Surface Tension (mN/m) Polytetrafluoroethylene 19Polydimethylsiloxane 24 Silicone Rubber 24 Polybutadiene 31 Polyethylene31 Polystyrene 33 Polypropylene 34 Polyester 39-42 Polyacrylamide 35-40Polyvinyl alcohol 37 Polymethyl methacrylate 39 Polyvinyl chloride 39Polysulfone 41 Nylon 6 42 Polyurethane 45 Polycarbonate 45

Preferred thermoplastic powders comprise urethane, carbonate, amide,sulfone, vinyl chloride, acrylate, methacrylate, vinyl alcohol, ester oracrylamide moieties. Useful thermoplastics (from which a powder can bemade) in accordance with the present invention have a modulus of 1 to200 MegaPascal and an elongation to break in the range of 25% to 1000%,more preferably 50%-500% and most preferably 100%-350%.

If mixtures of thermoplastic materials are used, then at least about 20weight percent of the thermoplastic material is hydrophilic as describedabove, e.g., provides a critical surface tension greater than or equalto 34 milliNewtons per meter. The different thermoplastic materials canbe blended, and powders can be created from the blend. Alternatively,different thermoplastic materials can be made into powders individuallyand thereafter combined as a blend of dissimilar powders. By combiningdifferent thermoplastics, physical properties can be chosen to provideimproved processability, such as, mold release and suseptability tocutting (i.e., skiving). Other thermoplastics can be chosen to improvepad performance, i.e., improved hydrophilicity, improved elongation tobreak, improved resistance to plastic flow, etc.

Since the starting materials is a powder, the final product will have asubstantially continuous bulk porosity. In the processes of the presentinvention, a premeasured quantity of powdered thermoplastic polymer ispoured into a mold and tapped or vibrated to settle the particles. Thisyields a volume of particles in tangent contact with each otherthroughout the powder volume. A two-dimensional schematic drawing ofthis packed state is illustrated in FIG. 1. In the illustration,particles 1 are in tangential contact 2. An interparticle void is shownat 3.

For example, the particles might involve hexagonal close packing ofmono-dispersed spherical powder particles, which would provide atheoretical powder density of about 67%. However, many commerciallyavailable powders have measurable size variation and are generally notspherical. These differences will lead to a change in the solids densityof the powdered compact prior to sintering relative to the simplesttheoretical case.

While any size particle may be employed to produce a pre-sinteredcompact, the processes of the present invention preferably use particleshaving an average diameter in the range of 20-100 microns. Such anaverage diameter range is well suited for complex molds having finesurface detail to produce a macroscopically smooth final pad surfacewhich is free from large gaps or crevices. This improves the mechanicaldurability of the sintered product and improves the polishingperformance of the surface.

As sintering proceeds, plastic flow at the particle boundaries leads toparticle coalescence and a corresponding shrinkage of the interparticlevoid volume. In the present invention, the time and temperature used forsintering at a given applied pressure are specifically controlled so asto retain a fully interconnected void volume, i.e. sintering is notallowed to proceed to completion. Proper settings to achieve a desiredsintering density can be determined from trial sinterings within thepressure and temperature limits set forth herein. Pressures in excess of0.70 MPa are preferred.

The molds employed to produce products of the present invention may beof any size, shape and pattern desired. Critical features of the moldsare the dimensional accuracy of the internal surface and the temperatureresistance and rigidity of the materials employed. A preferred molddesign for controlling final pressure and sintered product thickness,illustrated in FIG. 2, employs a movable top plate 4 and a rigid bottomplate 5 which has a recessed ledge or press stop 6.

In practice, a premeasured quantity of thermoplastic polymer powder isintroduced into the bottom mold cavity 5 and tapped or shaken to createa densified powder compact 7. The top plate 4 is then placed on top ofthe mold, inserted into a constant temperature oven and heated to thedesired sintering temperature under pressure, the pressure being appliedto the top cover 4 by a piston 8 shown in FIG. 3. As sinteringprogresses the powder compact volume decreases until the top mold coverrests on the polishing stop 6. Pressure is retained for the desiredlength of time, the mold is cooled, and the final part 9 of preciselydetermined thickness 10 is removed.

Although a wide variety of thermoplastic materials are commerciallyavailable and usable as starting materials in the present invention, therange of utility may be considerably enhanced by employing mixtures oftwo different thermoplastic powders. By intimately mixing two materials,composite structures may be produced which have mechanical propertieswhich may be different than either material individually, and dissimilarmaterial mixtures may be produced from materials which cannot besynthesized directly due to material incompatibility. Of particularutility is the use of a mixture wherein one of the components has alower melting point than the other. When such a mixture is processed bythe present invention at a temperature not to exceed the melting pointof the lower melting component, sintering may be effected withsignificantly less chance of distortion, and is thus preferred.

Particularly preferred combinations of particles include mixtures ofparticles containing polyurethane with particles containingpolyethylene, polypropylene, nylon, polyester or a combination thereofThe polyurethane particles can provide advantageous pad properties(e.g., modulus, elongation to break, critical surface tension, etc.) andthe other particles have been found to be particularly useful inimproving processability, since sintered polyurethane particles can bedifficult to remove from a mold or can be difficult to cut or skive to adesired dimension. In one embodiment, at least about 10 weight percentof the particles comprise polyurethane, more preferably at least about20 weight percent and yet more preferably at least about 50 weightpercent and most preferably at least about 65 weight percent of theparticles comprise polyurethane. A preferred particle to be mixed withthe polyurethane particles comprises polyethylene.

The distinctive features and advantages of the present invention can befurther understood by studying the following examples, which are notmeant to be restrictive in any way. Through the study of these examplesand the above description, other uses and applications will becomeapparent to those skilled in the art.

EXAMPLE 1

Samples of several different thermoplastic polymers includingpolyurethanes (Texin 480A, Texin 455D, Texin 470D and Texin 970Dmanufactured by Miles Inc., Pittsburgh, Pa., and Isoplast 302manufactured by Dow Chemical Co., Midland, Mich.)) as well as Nylon 66were cryogenically milled into powder. The mean particle diameter of thepowder was 50 microns. Melting temperatures of the powders were measuredusing a Fisher-Johns melting point apparatus. Melting point data isgiven in Table 1.

TABLE 1 Melting points of polymer powders Material Melting point (° C.)Texin 470D 230 Texin 970D 210 Isoplast 302 200 Texin 455D 230 Texin 480A225 nylon 66 260

Sintering tests were conducted on each of these materials at varioustemperatures using a 12 in. diameter press mold of design shown in FIG.2. The stop depth selected was 0.062 in. for a total mold depth of 0.125in., allowing 2:1 compaction. Dimensional tolerancing of the mold cavitywas ±0.001 in. Samples were pressed by first filling the mold cavitywith powder in a uniform fashion, gently vibrating the powder, andscraping off excess powder in the mold to ensure that the entire volumeof the mold cavity was uniformly filled with starting material. The topportion of the mold was then placed onto the powder fill and the entiremold assembly placed in a heated press at room temperature and 150 psi(1.03 Mpa) pressure applied to the top portion of the mold. The entireassembly was then heated to the desired temperature and held for 20minutes to effect sintering. At this point pressure was released and themold removed and allowed to cool to room temperature before removingproducts for examination.

In all cases, powder pressed at room temperature showed essentially nosintering. Samples pressed at temperatures above the melting pointshowed nearly complete sintering to a dense non-porous solid. Asignificant degree of adhesion to the mold was also observed. Incontrast, for all materials tested, a temperature range of 175-200 Cyielded a strong resilient sintered product which did not exhibitadhesion to the mold. Examination of products sintered in this regionshowed a high degree of internal porosity and good interparticlesintering. All products sintered in this temperature range showed goodair and water permeability. Sintered pad thickness in all cases was0.062 in, exactly corresponding to the mold stop depth. Dimensionalvariation was ±0.001 in, again precisely corresponding to the moldsurface dimensions and thickness. Surface quality of the products showedthem to be extremely smooth and uniform; comparable to commercialpolishing pads.

EXAMPLE 2

Several mixtures of plastic powders were processed using the procedureoutlined in Example 1. A sintering temperature of 200 C was employed.Mixtures tested are listed in Table 2 below.

TABLE 2 Powder mixtures used in sintering tests Component 1 Component 2Component 3 Texin 470D 50% Isoplast 302 50% Texin 470D 20% Isoplast 30280% Texin 470D 80% nylon 66 20% Texin 470D 50% Texin 970D 50% Texin 470DIsoplast 302 33.33% nylon 66 33.33% 33.33%

All sintered products showed good flexibility, strength, dimensionalprecision and porosity, fully equivalent to the best single materialsamples of Example 1.

EXAMPLE 3

Another top mold plate was prepared which had a series of concentricprojecting rings on its inner surface. Ring spacing was 0.030 in, with aprojecting depth of 0.015 in and a projection width of 0.013 in. Thistop plate was substituted for the original top plate and used to sintersamples of 970D powder using optimal conditions identified in Example 1.The resulting product had a top surface which had a pattern ofconcentric circular grooves of a precise mirror image of the projectingconcentric circular grooves of the mold surface. Dimensions anddimensional precision were found to be equivalent to the mold, as in theother examples. All portions of the product, including the regionsbetween grooves on the top surface were of uniform porosity.

EXAMPLE 4

A sintered product pad made from 470D polymer using the proceduresoutlined in Example 1 was tested for planarization polishing activityand results compared to a commercially available polishing pad, IC1000(Rodel, Inc.), which is currently widely employed as a planarizationpad. Experimental conditions used are given in Table 3 below.

TABLE 3 Parameter Setting polisher Strasbaugh 6CA table speed 100 rpmspindle speed  60 rpm load  7 psi slurry used ILD1300 silica slurryslurry flow rate 100 ml/min polish time  2 minutes pad conditioning nowafer type  4 in. diameter thermal oxide on Si

The IC1000 pad showed an initially high polishing rate of 1300 Å/min.which decayed steadily to a lower rate of 550 Å/min by the twentiethwafer processed. This is graphically illustrated in FIG. 4. In contrast,the polishing pad of the present invention showed considerably improvedrate and rate stability. The initial polishing rate observed was again1300 Å/min. This decreased to a constant rate of 950 Å/min by thetwelfth wafer. This is graphically illustrated in FIG. 5. Thus the padof the present invention exhibited both significantly increased ratestability and increased rate.

EXAMPLE 5

A sintered product pad made from 455D polymer using the proceduresoutlined in Example 1 was tested for Silicon polishing activity.Experimental conditions are summarized in Table 4 below.

TABLE 4 Parameter Setting polisher Strasbaugh 6CA table speed 100 rpmspindle speed  60 rpm load  7 psi slurry used Naleo 2350 slurry flowrate 100 ml/min polish time  20 minutes wafer type  4 in. diameter [110]Si

A series of 25 wafers were polished. Polishing rate was initially 0.4microns/min, and rapidly increased to a constant value of 0.8microns/min. The polishing rate obtained was comparable to that obtainedusing conventional Si polishing pads such as Suba IV (Rodel, Inc.).However, wafer flatness and surface quality as observed by Nomarskimicroscopy were markedly superior to results obtained with conventionalpads.

EXAMPLE 6

In an alternative method of fabrication, a blend of 70 weight partsTexin 470D polyurethane and 30 weight parts polyethylene powders wereformed into a cylindrical billet, under carefully controlled conditionsof temperature and pressure.

The billet was subsequently skived into a 1.3 mm thick sheet from whicha polishing pad was cut. The pad was used to polish silicon wafers. Thepolishing conditions were:

Polisher Strasbaugh 6CA Table Speed 200 rpm Spindle Speed  80 rpm Load 48 Pa Slurry Rodel 1540 Slurry Flow Rate 180 ml/min Polish Time  20 minWafer  6 inch diameter silicon

The pad was initially conditioned for 5 minutes, using a 100 gritdiamond conditioning disk. A stable removal rate of 1 micron per minutewas achieved without the need for additional conditioning betweenwafers.

EXAMPLE 7

Sintered compacts of thermoplastic polyurethanes can also be fabricatedfrom aqueous slunies of thermoplastic polyurethane powders. In somecases, small amounts of isopropanol and/or water soluble polymers, suchas poly(vinyl alcohol), can be added to improve rheology and thestrength of the compact prior to sintering.

Powdered Texin 470D (100 g) was mixed with deionized water (144 g) togive a paste. This was poured into a circular mold and dried at 100 Cfor 6 hours. After drying the sample was pressed in the closed mold at1.03 MPa and sintered at 180 C for 25 min to give a pad having a densityof 0.8 g/cm3 and 35% porosity.

EXAMPLE 8

Powdered 470D was mixed with a poly(vinyl alcohol)/water/isopropanolsolution to form a paste which was poured into a circular mold as shownin FIG. 2. After drying at 100 C for 6 hours, the compact was cohesive.The compact was then pressed in the mold at 1.03 MPa and sintered at 185C for 25 minutes to give a sintered pad containing 1 wt % poly(vinylalcohol). The pad appeared to have uniform packing density. Density andporosity were 0.9 g/cm³ and 27% respectively.

What is claimed is:
 1. A polishing pad comprising: a polishing surfacehaving a plurality of sintered thermoplastic particles, the particlescomprising a thermoplastic material having a critical surface tensiongreater than or equal to 34 milliNewtons per meter, a modulus of 1 to200 megaPascals and an elongation to break in the range of 25% to 1000%.2. A pad in accordance with claim 1, wherein the thermoplastic materialcomprises urethane, carbonate, amide, sulfone, vinyl chloride, acrylate,methacrylate, vinyl alcohol, ester or acrylamide moieties.